The invention relates to a so-called water heater, in which a liquid heat carrier is present as the heat exchange medium. In particular, the invention is directed to a vehicle auxiliary heater of the type having a burner in a combustion chamber which is surrounded by a heat exchanger which is, in turn, surrounded by a jacket casing to form a jacket space between the heat exchanger and the jacket casing through which the liquid heat exchange medium, such as water, flows, and with a temperature safety mechanism that shuts off the heater to prevent damage to temperature-sensitive parts.
To protect the heat exchanger and other temperature-sensitive parts from damage by overheating which, for example, may occur if the heater is started without liquid heat carrier or if, during operation of the heater, a temperature sensor acting as a control thermostat for regulating the heater operation fails, previously, an immersion type overheating temperature sensor is used in a heater of the type mentioned above that extends through a through-hole in the jacket casing and is suitably fixed in place there. For example, in German Pat. No. 30 25 283, the head of the overheating temperature sensor goes through the jacket space, through which the liquid heat exchange medium flows, and rests on an outer wall surface of the heat exchanger. The outlet of the immersion type sensor is connected to a control device of the heater so that the heater is automatically shut off if the immersion type temperature sensor supplies an output that exceeds a predetermined value. The immersion type sensor must be placed in an especially liquid-tight manner within the bore hole of the jacket casing, for which suitable seals are necessary.
If an immersion type sensor, which acts as a temperature safety mechanism, is to be replaced because, for example, it is defective, or if the heater has been automatically shut off by the temperature safety mechanism with the aid of the immersion type sensor, the immersion type sensor must be taken out of the opening in the jacket casing, so that the fluid-tight sealing of the liquid heat exchanger is broken. During replacement, the liquid circuit of the liquid heat carrier in the heater is thus opened and, for this purpose, it is necessary, if the heater is connected, for example, to the coolant circuit of the motor vehicle, to uncouple the heater from the coolant circuit of the vehicle before the immersion type sensor is taken out. After replacement and installation of a new immersion type sensor, the connection to the coolant circuit of the motor vehicle must, for example, again be made. Thus, the replacement of such an immersion type sensor is expensive, complicated and time-consuming, so that the repair costs needed for this are high.
Furthermore, the known immersion type sensor that contacts the outer wall surface of the heat exchanger is also problematic from the standpoint of reliable operation. That is, by contacting the heat exchanger, a more accurate reading of temperature conditions can be obtained. However, the heat exchanger and surrounding housing jacket will expand and contract to differing extents as a result of the different and varying temperatures to which they are exposed, so that constant contact between the immersion type sensor and the heat exchanger outer wall surface cannot be assured without creating an unacceptable risk of damage due to thermal expansion/contraction effects. Thus, since temperature within the liquid flow-through space of the heater varies with radial distance from the combustion exhaust gas space bounded by the inner surface of the heat exchanger, reliably accurate and sensitive overheat protection cannot be obtained with this known arrangement.